Whatever the fate of the Carter administration's proposed gas tax, it is an established fact in Italy. And Italian drivers have discovered that steep gasoline taxes alter both their driving habits and lifestyles. The result has been a marked decline in the consumption of gasoline.

A two-stage, 42 per cent price increase put into effect by the Italian government in 1976 in the hopes of reducing the country's $7.5 billion oil deficit caused a 5 per cent drop in consumption during that 12-month period.

Officials at the Oil Producers' Union in Rome believe, however, that the drop in gas sales this year will be even greater, perhaps as high as 7 per cent. And they point to an 11.3 per cent drop in consumption during the first three months of 1977 as an indication of the current trend.

In real terms, however, the drop is even greater, they say. For while gas sales fell in 1976, total number of cars in circulation increased by more than 1 million in the same period. The average Italian cut gasoline comsumption by more than 9 per cent from 1975 to 1976.

According to a spokesman at the producers' union, a comparison between gas sales and the increase in an average decrease in gas use per vehicle of more than 20 per cent.

"People are still buying cars and still using them, but they are using them in a different way," he said. "Most motorists have eliminated long car trips in favor of travel by train and plane. The average family still clings to its famous Sunday outings," he admitted, "but surveys show they just don't go so far." At present, more than 70 per cent of the gas sold here is consumed within city limits where notorious traffic jams have eased only slightly.

Since the explosion of the oil crisis in the fall of 1973, the price of gasoline in Italy has been raised seven times from the September 1973 level of 185 lire for a liter (1.1 quarts) of super to the current all-time high of 500 litre (about 57 cents).

This is equivalent of $2.56 a gallon, the highest price in Europe. "In concrete terms, it means I have to spend $24 every time I fill up the tank of my Volkswagen Beetle," a foreign resident here said recently.

Several of those seven price increases - for example, those in November 1973, February and July 1974, and November 1975 - represented price adjustments for Italy's badly hit oil producers as well as partial tax hikes by the Italian government.

Since 1960, Italy has collected betrween 60 and 77 per cent of the final gasoline price in taxes. But the most recent increases, in March and October of last year, for a total of 150 lire (17 cents) per liter, were imposed exclusively to induce Italians to buy less "benzina."

"We consider our overrall policy a success," said an official at the Ministry of Industry. He pointed out that since 1973 the country has reduced total imports of crude oil by about 28 million tons. All this could not be attributed to the decline in gas consumption, he said, "but because of the refining processes involved in the production of gasoline from crude oil, it certainly has played an important part."

A breakdown of the current 500-lire-a-liter price tag shows in any event that only 143.93 lire of that amount goes to the producer. The government takes the rest in the form of a variety of added value, manufacturing and other taxes.

This amounts to 71.2 per cent, a considerably higher percentage than the taxes on gas collected by Western Europe's other government.

After the outbreak of the oil crises, the Italian government sought a cut in consumption through a variety of methods, including a modest price increase, the temporary banning of driving on Sundays, and the broadening of pedestrian zones in the downtown areas of most major Italian cities.

In a country where the automobile remains the major sign of proposperity and status, however, the first results were disappointing.

After an inital drop in consumption of 8 per cent in 1974 gas sales climbed by 7 per cent in 1975. The government came to the conclusion that something more was necessary.After prolonged discussion of the defects and merits of a rationing system based on a double-tier price system, it was decided that only a major price increase could have the desired effect.

"And they were right," said an official of the Italian Automobile Club, who thinks, however, that a rationing system would have been more fair and less regressive.

"With a liter of gas at 500 lire, people really stop and think before using their cars for long-distance travel," he said. "They'd probably use them even less if our major cities where most of the driving is done, had more efficient systems of public transport."